Optimization of integrated microalgal biorefinery producing fuel and value‐added products

Abstract

AbstractAn integrated microalgal biorefinery is desirable from an economic standpoint but challenging to synthesize, due to diversity of options. This work uses a model‐based optimization approach to address this challenge in a systematic manner. A superstructure of the integrated biorefinery is developed where biodiesel is considered as the main product, while polar lipid, protein, and carbohydrate are also processed to various value‐added compounds. Mass balances, equipment capacity limitations, and cost functions corresponding to these processes constitute the constraints of the optimization problem. The decision variables include the process synthesis as well as process scheduling and operations‐related decisions. A Mixed Integer Linear Programming (MILP) model was developed to minimize the net annualized life cycle cost (ALCC) of the biorefinery. For a scenario of 30 Mg/d production target of biodiesel, with no intermediate storage between the stages, the superstructure yielded an optimal biodiesel production cost of US$8.53/L and reduced sugar was selected as a co‐product. Several cases were analyzed in terms of the decision making of the process on the upstream and downstream levels, as well as variations in scheduling strategies. Co‐cultivation of the phototrophic and heterotrophic strains resulted in net ALCC of US$7.66/L, which was 10.2% less than the base case. Batch scheduling with various strategies were also investigated and the case with infinite intermediate storage coupled with debottlenecking reduced the net ALCC by 25% to US$6.4/L. © 2017 Society of Chemical Industry and John Wiley & Sons, Ltd